The NLSY doesn't include a test of IQ. It includes an AFQT test. That stands for Armed Forces Qualification Test. The NLSY tracks young people who are between 14-22 in 1979 through time. Therefore, they take the AFQT test when they are at least 14 and often older. The AFQT test isn't designed to measure intelligence per se. Your argument is pretty much that students who score well on standardized tests in high school and college are much more likely to graduate (or to have graduated) from high school. There are all sorts of correlation/causation problems there.